The invention relates to a method for detecting a leak in a heat-recovery system of an internal combustion engine of a motor vehicle, wherein the heat-recovery system comprises at least one combustible working medium, a working medium circuit with at least one EGR evaporator, a pump and at least one expansion machine.
When operating a system for heat recovery with a combustible working medium in conjunction with an internal combustion engine and an exhaust gas recirculation evaporator (EGR evaporator: EGR=Exhaust Gas Recirculation), the detection of leaks in the system is of high priority. Leakages in a heat-recovery system can lead, among other things, to the following critical scenarios:                Discharge of the working medium into the environment—leads to the risk of fire when using a combustible working medium such as ethanol.        Entry of the combustible working medium into the internal combustion engine, which causes damage when, for example, the working medium passes into the combustion chamber via an EGR evaporator.        Overheating of system components due to insufficient working medium level, which, for example, can lead to overheating of the exhaust gas evaporator if the mass flow of the working medium is too low.        
To detect a leakage in a heat-recovery system, for example, the following methods are known:                Monitoring of the filling level of the working medium in the expansion tank by means of a level sensor. If the filling level is too low, a leak is assumed.        Leak test by pressurizing the deactivated cold system and then observing the pressure gradient. A too rapid pressure drop indicates a leak.        Measuring the electrical conductivity of the insulation of the heat-recovery system. A change in conductivity is a sign of a leak.        
U.S. Pat. No. 6,526,358 B1 describes a method for the detection of leaks and blockages in a fluid circuit, wherein pressure, temperature and flow rate at different points of the circuit are measured and correlated.
JP 2010-156314 A discloses a heat-recovery system for an internal combustion engine, wherein O2 sensors are arranged for leakage detection in the coolant circuit of the heat-recovery system.
Known methods have the disadvantage that they can either be performed only in the deactivated state and/or that devices such as additional sensors or the like are required.